While Mr Clegg insisted that the shake-up does not represent a return to a two-tier system of education, Christine Blower, general secretary of the National Union of Teachers, said that this is "blatantly" not the case.

"Pupils who do not gain EBacc Certificates will receive a record of achievement which will most certainly be seen to be of far less worth by employers and colleges," she said.

Ms Blower also expressed concern about the effect the changes would have on those with jobs in education and called for more engagement with secondary teachers.

"Ministers need to recognise that it is through sheer hard work and determination that pupils and teachers are achieving great results," she said.

"Teachers are not mere 'deliverers' of knowledge, but are there to inspire and motivate their students with a curriculum that is for everyone and not just for the few."

Chris Keates, general secretary of the National Association of Schoolmasters Union of Women Teachers, was similarly cautious about the new proposals.

She is worried that because the EBacc system is focused on the 'core' subjects (English, science and maths) other areas of the curriculum could be marginalised.

"Parents will continue to want to support a system which supports excellence for all in music, drama, RE, PE, ICT and other creative subjects," she said.

The move away from coursework is also an area of concern, she added.

"The secretary of state must avoid the dangerous view that terminal examinations have intrinsically more merit than coursework."